1. Field of the Invention
The present invention relates to plasma process system and method suitable for etching, ashing, sputtering and CVD-processing semiconductor wafers.
2. Description of the Related Art
When plasma is generated in an air-tight process chamber to etch the semiconductor wafer in plasma atmosphere thus generated, "voltage of sheath" is a parameter to check etching reaction because it gives great effect to the energy of ions. Fundamentally, it is determined by potential V.sub.p of plasma and negative potential V.sub.DC biased to the side of the cathode. It is therefore asked that these potentials V.sub.p and V.sub.DC are adjusted to adjust plasma when plasma is to be generated in the process chamber.
Conventionally, an apparatus in which high frequency power is added to one of upper and lower electrodes and the other of them is earthed is used to generate plasma. Another apparatus in which one of two high frequency power sources is connected to the upper electrode and the other is connected to the lower electrode is also used.
In any of these conventional cases, the upper and lower electrodes and the high frequency power source arranged outside the process chamber form a series circuit when the process chamber is regarded as a circuit element.
When plasma voltage is to be adjusted, therefore, it is difficult to independently and freely adjust only V.sub.DC in the apparatus in which the high frequency power source is connected only to one of the upper and lower electrodes.
Even in the case where the high frequency power sources are connected to the upper and lower electrodes, adjustment is conducted in a series circuit. The high frequency power sources used, therefore, must has same power and same capacity. This makes the apparatus large in size.
In the case where plasma is generated in the process chamber and the semiconductor wafer is processed in plasma atmosphere thus generated, three main steps are applied to the wafer. They are anisotropic isotropic etching steps and a discharge step. The discharge step is intended to discharge the charged-up electrostatic chuck. The wafer once attracted by the electrostatic chuck, sometimes, is not released from the chuck even when the power source is switched off. This is because load caused from electrostatic polarization causes the electrostatic chuck to still attract the wafer. The discharge step is therefore needed to solve this problem. To carry out the discharge step, however, weak plasma must be generated. The wafer is thus unnecessarily etched and damaged in this case.
On the other hand, there has been desired a system in which all of the anisotropic and isotropic etching steps and the electrostatic chuck discharge step can be conducted only in a process chamber.
Jpn. Pat. Appln. KOKAI Publication No. Sho 56-84476 discloses a method of using high frequency power having a frequency lower than 10 MHz and placing the wafer adjacent to the frequency electrode arranged on the high voltage side (or plasma etching method of the parallel plate type). It further discloses a cathode and anode coupling method by which power source and earthed electrodes are switched over between them.
Jpn. Pat. Appln. KOKAI Publication No. Hei 1-253238 discloses an apparatus in which high frequency voltage can be added from a high frequency power source to both of upper and lower electrodes at the same time and to one of them independently of the other by means of a changeover means.
In these conventional method and apparatus, however, all of the anisotropic and isotropic etching steps and the discharge step cannot be conducted in a process chamber. Every step asks a chamber in these cases. Therefore, they occupy a large space in a clean room. Particularly when matters to be processed become large in size like an 8-inch wafer and an LCD substrate, an extremely large system is needed. In addition, the conventional system has a long wafer carrying passage. This means that their throughputs are low and that they causes so many particles and others to adhere to the wafer.
In order to reduce the amount of particles and others adhering to the wafer, single or plural load lock chambers are located adjacent to the plasma etching chamber. When their internal pressures are to be returned to normal, it usually takes a relatively long time (or about one minute, for example) because nitrogen gas must be gradually introduced into them to prevent particles and others from floating in them. This asks the wafer to be kept waiting for a long time when it is to be carried into and out of them. The throughput is thus made low.
In the conventional magnetron plasma process system, plasma is maldistributed, because of magnetic field, in the process chamber, particularly along the inner circumference of it, even if plasma is made uniform by scanning magnetic field. The rim portion of the wafer is thus charged up, thereby causing age oxide film to be dielectrically broken down at the rim portion of the wafer. The rate of dielectric breakdown reaches about 10% of all products and the productivity of the system is thus made low.